78 ALASKAN GLACIER STUDIES
Thus, proceeding down the glacier surface, one sees, first, barren ablation moraine with no vegetation, then ablation moraine with crescentic ridges and valleys littered with dead alder, and finally ablation moraine covered with dense alder thickets undisturbed by the advance. No evidence as to the extent of lowering of this part of the glacier was seen, but the photographs show clearly that the western portion of the piedmont bulb, as observed from Terrace Point, had been greatly lowered, and that a series of crescentic ridges and valleys had developed in the moraine-covered outer portion. The amount of lowering here is comparable to that observed elsewhere. The crescentic breaking, and the ridges and valleys which have succeeded it, present the appearance of viscous flowage of an underlying ice stream, carrying with it and breaking up a rigid upper crust of ice.
One of the most notable features on the Atrevida Glacier, as seen in 1909, was a large, roughly-crescentic area of clean white ice, with only a few short ridges of debris upon it, just outside the mountain front (PI. XXV, B). Upstream from it lies the valley portion of the glacier which is almost completely covered with debris; and downstream from it lies the area of crescentic crevassing of the piedmont bulb, already described. This clear ice area extended westward nearly to Terrace Point, broadening in that direction and occupying a part of the bulb which protrudes toward Lucia Glacier—the part of the Atrevida in which there was the greatest thickening and forward movement. In 1905 the site of this area of clear ice was occupied by moraine-covered ice, but in 1906, (Pis. XXVI, XXVII) as already mentioned, it appeared in the broken glacier surface as an area of somewhat-domed, clear ice (PL XXV, A) surrounded by broken, partially-de"bris-covered ice. On its inner, or up-stream, side the glacier surface rose abruptly in 1909, as a high wall covered with moraine, and above this the glacier was almost completely covered with ablation moraine. The clear area was fully a mile and a half long (from east to west) and half or three quarters of a mile wide in the middle, which is the widest part, narrowing toward both the east and the west. It had spread somewhat in the interval between our photograph of 1906 and the period of observation in 1909. In 1909 moraine-covered ice rose above the clear area on the down-stream side and on both ends as well as on the up-stream side, but the change was less abrupt and pronounced in these directions than up-stream. The clear ice area formed a distinct depression in the glacier surface. Down-stream the clear ice area gradually died out, bands of debris appearing in it. The fact that this area was lower than the surrounding debris-covered ice is probably the result of more extensive ablation in the clear part of the glacier; and that there had been much ablation here is further proved by the fact that most of the crevassing, by which the area, was severely broken in 1906, had disappeared. Many shallow crevasses, broadened by melting, still remained, but they were only the remnants of the former extensive gashes,—the crevasse bottoms. The surface was quite rough, but travel over it was not difficult, only short detours being necessary to avoid the crevasses (PI. XXIV, B), Although not directly on the route it was chosen as the easiest path from Amphitheatre Knob to Terrace Point.
In seeking the explanation for such a peculiar area,of clear white ice in the midst of a waste of ablation moraine we are aided by four noteworthy facts: (1) it has appeared as a result of the 1906 advance; (2) morainic debris that covered the ice on this site in 1905 had disappeared, for practically none remained even in the crevasse bottoms; (3) the clear ice area was distinctly domed in 1906; (4) some, at least, of the layers of ice in the